Production of paper pulp



Aug, 12, 1930. R. B. WOLF 1,772,792

PRODUCTION OF PAPER PULP Filed June 26, 1929 ATTORNEYS Patented Aug. 12, 1930 1103mm: 3. worn, on NEW YORK, N. Y.

PRODTICTION OF PAPER PULP Application filed June 26,

This invention relates to the production of paper pulp and particularly to improvements in the sulphite process and in the recovery of the acid employed therein.

To secure a proper understandin of the invention it is necessary to review briefly the development of the sulphite process for treating paper pulp which was described original- 1y in the patent to Tilghman No. 70,485,

November 5, 1867. In the process as originally applied, it was impossible to recover practically the blow-01f or relief gases. These gases, consisting principally of sulphur 'dioxide, were discharged to the atmosphere and constituted a serious loss. The first attempt to recover the gases" involved the ab- X sorption thereof in the cooking liquor, as described in theDrewsen Patent No. 492,196 dated February 21, 1893. This constituted a material advance, but the method was subject to certain disadvantages, particularly the impossibility of retaining the sulphur dioxide gas in the acid at the high temperature resulting from the introduction of the hot gas thereto.

The next improvement involved the introduction of so called relief coolers between the digesters and the recoverytanks, so that the temperature of the acid could be kept relatively low, usually between 70 and 90 F. -With such improvements it was possible to recover the acid at a temperature of approximately 75 F. and at atmospheric pressure containing approximately 7% of sulphur dioxide, although in the ordinary mill the total sulphur dioxide content of the acid averages between 4 and 5%. i

The advantage of using strong acid was pointed out in the Wolf Patent No. 1,699,556

. dated June 22, 1929. That patent also called attention to the fact that strong acid has not been used because the excess of sulphur dioxide gas remaining inthe digester at the end .of the cook ordinarily could not be forced over into the recovery tanks without advancing the temperatures in the cooking operation and thereby losing most of the benefit resulting from the use of strong acid. -A' method of recovering strong acid is described 5 in the Wolf Patent No. 1,097,781 dated May 1929. Serial No. 373,786.

26, 1914, and a simpler and more efiective method is described in the Wolf Patent N 0. 1,499,898 dated July 1, 1924.

.To get the best results, strong acid should be used at low temperatures. This principle is recognized inthe patent to Moore and Wolf No. 1,110,454 dated September 15, 1914, C which relates to a method of blowing the di gestersinto ablow pit and collecting the vomit stack gases and passing them through a surface condenser to condense 'out the water vapor, the acid being prepared from the recovered gases. A simpler method of accomplishing the same result is described in the Wolf Patent No. 1,699,556.

Where strong acid is used, a much larger volume of gas must be relieved from the digester. This necessitates the use of very much larger cooling equipment for cooling the re lief gases before they pass to the recovery tank The cooling equipment is expensive to maintain because lead must be used on account of the corrosive action of the sulphur dioxide gases in the presence of moisture and because of the large relief lines necessitated by the volume of gases.

Another step in the industry was in the direction of making it possible to get the effect of stronger free acid by another method, namely, the method of relieving the digesters into an accumulator without cooling the relief gases and liquids. The first description of this method appears in the Clemm Patent No. 1,429,128 dated September 12, 1922 which is followed by the hot acid recovery system of Richter described in Patent No. 1,653,416 dated December 20, 1927 and in other and later patents.

In the hot acid recovery system the emphasis is upon the recovery of heat from the relief gases. These gases are conducted, therefore, without cooling to a hot acid accumulator. .In the ordinary mill, a number of digesters are connected to a common relief line which carries the gases from the I digesters to the acid accumulator. This induces serious difliculties including a wide variation in the acid strength and in the temperature of p the acid which is introduced from the accumulators to the digesters, but

i of the pressure in -tion of a digester.

the digester.

more particularly the relief line which seriously affects the cooking operation, being equivalent to constant manipulation of the relief valves of the respective digesters. It is a matter of common knowledge in the operation of digesters that the pressure therein must remain substantially constant. The operator usually sets the relief valve at the desired opening tomaintaina constant pressure, but this control becomes wholly ineffective under the system described in which the hot relief gases are conducted through a common relief line to.the hot acid accumulators.

To appreciate this difficulty more fully, it is desirable to consider the normal opera- The digester is first filled with chips and with acid, the acid being, as nearly as possible, of uniform temperature and uniform chemical composition. The cover of the digester is secured in place, and steam is admitted into the bottom of 'lhe pressure is brought up to 7 5 pounds per square inch at the bottom of the digester in a period ranging from two to three hours. After the pressure has reached 7 5 pounds, which is the usual working pressure, no more steam can be admitted unless gas is relieved from the digester. This is accomplished by means of a relief valve on the cover. The speed with which the temperature in the digester is raised depends upon the amount of opening of the relief Valve, a wide opening, permitting the escape of a large amount-of gas, lets in a large amount of steam and therefore there will be a rapid advance in temperature. The reverse is true if the relief valve is only slightly open, namely; the temperature will advance slowly. It is possible to control the speed of cooking, so that the variation in practiceis between seven hours and twenty hours, the purpose of the longer cooking time being to complete the disintegration of the wood chips into fibres at as low temperatures as is possible. .If the pressure in the relief line varies constantly, the control of the cooking time is lost substantially and approximately the temperature of the digester will fluctuate,

widely because of the variations in the amount of steam introduced thereto.

The rate of acceleration of temperature in the cooking operation is very important. In other words, it makes quite a difference in the character of the pulp whether the tem perature is advanced rapidly during the first part of the cook and slowly during the latter part, or advanced slowly during the first part and rapidly during the latter part of the cook. The rate of advance of tempera ture is entirely amatter of control of the relief valve on the cover of the digester; hence it 1 s very important for the pressure in the relief lines to be absolutely constant so that in the constant variation when the cook sets his valve in a certain position he knows he will get a constant amount of relief.

It is the object of the present invention to provide a method and apparatus whereby the difficulties experienced heretofore in the operation of digesters and the recovery of acid therefrom are avoided and whereby stronger acid can be recovered and utilized more effectively and without losses heretofore incident to the operation of the sulphite process.

Other objects and advantages of the invention will be apparent as it is better understood by reference .10 the following specification and the accompanying drawing, which illustrates diagrammatically the preferred form of the invention. In carrying out the invention, I employ two or more recovery tanks and a small relief accumulator, the latter being connected preferably by separate relief lines to each individual digester of a group constituting the system. The relief accumulator is a small vessel adapted to receive the gases and liquids from the individual digesters as they escape through the relief valves thereof. The gases and liquids are permitted to escape from the accumulator through a pressure regulating valve, preferably of the type known as the Locke regulator which is used in sulphite pulp mills to maintainuniform pressures in the digesters during the cooking operation. By the use of this valve a constant pressure is maintained in the relief accumulator, and consequently the relief lines leading thereto are likewise stant pressure so that all'variations in the relief lines, which otherwise would affect the cooking operations in the digesters, are avoided.

From the relief accumulator the acid liquids and gases are conveyed into one or the other of the two recovery tanks and are therein subjected to cooling to a predetermined temperature. The cooling may be effected by acoil immersed in the acid solution in each recovery tank, or the acid may be withdrawn from each of the tanks, circulated through a cooling device, and returned thereto. The purpose of cooling the acid is to maintain a relatively low temperature, thus permitting the absorption and retention of a larger proportion of sulphur dioxide gas at the pressure maintained in each of the remaintained at a conwith a very Wide margin.

Referring to the drawing, 5 indicates the digesters of a group, each of which is connectedby a relief line 6 to the relief accumulator 7. The latter is a small tank adapted to Withstand the pressure of the gases delivered thereto from'the digesters. The relief lines may be connected thereto in any suitable manner, being arranged, for example, about the periphery of the cylindrical tank. A pipe 8 extends substantially to the bottom of the relief accumulator and is provided with openings9 which permit both'gases and liquids to enter the pipe which serves as an outlet therefor. A pressure regulating valve 10 is disposed in the pipe outside of the accumulator to maintain a constant pressure therein. I prefer touse a Locke regulator which consists of a diaphragm 11 connected by a pipe 12 to the top of the relief accumulator. The diaphragm is counter-balanced by a weight 13 to permit predetermination of the pressure to be maintained. The diaphragm is also connected to a pilot valve 14 which controls the water pressure in a cyl inder 15. A piston 16 in'the cylinder is connected to the operatinglever 17 of the valve 10. 7 Thus, as the pressure tends to vary in the accumulator, the valve 10-is opened or closed and a uniform pressure is maintained.

As both gas and liquid are relieved from the digestor and it is desirable to have a fairly uniform mixture of the gas and liquid passing from the relief accumulator continuously, the pipe 8 is provided with perforations as described. This permits the maintenance of a gas space in the top ofthe accumulator.

' As .the liquid level rises, the gas pressure will in effect atomizethis liquid and force it out through the regulating valve. The .gas and atomized liquid are directed through either of the pipes 18 and 19 controlled by valves 20 and 21 into the recovery tanks 22 and 23. The latter are large tanks capable of withstanding pressure and of holding large quantitiesof acid which may be supplied thereto from the acid storage tanks through inlet pipes 24 and 25. They are provided with safety valves 26 and 27 and with'pressure gauges 28 and 29, so that excess pressure may be relieved and the condition of the acid in the tanks may be readily determined. Outletsv 30., and 31 controlled by valves 32 and 33am provided to permit withdrawal of the acid and the delivery thereof to the respective digesters.

In'the tank 22 I have'shown a cooling coil 34 which may be supplied with cooling water from any suitable source by a pipe 35. The Water may be Withdrawn through a pipe 36. Suitable valves are provided to control the flow of Water through the cooling coil. The

purpose of the cooling coil is to reduce the temperature and to maintain the acid in the tank at a predetermined temperature. This is readily accomplished by modifying the flow of water through the cooling coil in accordance with the existing conditions and particularly the temperature of the gases entering the recovery tank. A recording thermometer 37 may be employed to indicate the temperature of the acid. Alternatively I may employ the cooling device illustrated in connection with tank 23, including a pump 38 connected by a pipe 39 to the tank and by a pump 40 to a cooler 41 from which the acid is returned through a pipe 42 to the tank. Normally one or the other of the cooling means described will be utilized in both of the recovery tanks.

In practice the operation will be substantially as follows: The pressure in the relief accumulator as the digesters are normally -cooking will be constant. The gas and liqnor will be conducted into either one or the other of the recovery tanks. as desired. If, for instance, the left hand recovery tank has just been emptied of its charge in order to till a digester. the right hand tank will be filled to a predetermined level with acid and will be receiving the relief gases. The temperature will be maintained either automatically or. by hand regulation at a constant point predetermined byexperience but in z the neighborhood of about 120 F. The relief Will be allowed to continue into this recovery tank until the tank has reached a definite pressure also predetermined and based upon operating experience, probably about 40 pounds per square inch. With a given temperature the pressure will be an exact indication of the acid strength. When this pressure has been reached, the opposite recovery tank will be put into use by opening the'proper valve leading into it and closing the corresponding valve of thetank in which the acid has reached the desired strength.

It can be readily understood that the control of temperature and pressure and consequentlyof the strength of the acid permits uniformity in the acid supply to the digesters which could not be obtained by any previous method, and that at the same time uniform- 1ty of back-pressure against the relief valves of the respective digesters is assured, so that the cooking is uniform and the quality of the pulp can be maintained.

The uniformity of pressure in the relief lines is essential only during the actual cookadvancement. When the period for blowing a down the pressure in' thedigester is reached,

that is to say, approximately an hour before the contents of the digester is blown into the blow pit, a connection into another relief line 43 can be made by actuating a suitable valve. Ordinarily this will be a common pipe enter the recovery tanks at about their middle portion, but they can be in any position which may proveto be convenient.

This procedure of lowering the pressure before the digester is blown may be used, but I prefer to keep the pressure on the digester up to the regular cooking pressure until the end of the cooking and then blow the pressure all off from the digester by the method described in my Patent No. 1,699,556.

The gas absorbed in the latter procedure willfind its way to the storage tank the pressure recovery tanks are filled before s from which the relief gases are introduced thereto. It is to-be understood that the storage tanks are supplied with fresh acid from the acid system, preferably the system described in my Patent No. 1,499,898 which permits the manufacture of acid much stronger than it can be made under the systems heretofore in use.

Among the advantages of the method as described are the recovery and storage of acid under predetermined conditions of temperature and pressure and consequently of acid strength, the improvement in cooking effected by the use of such acid, and particularly the avoidance of variations of back-pressure in the relief line which would otherwise interfere with the cooking operation. It is possible by employing the invention to operate more economically and to produce a much higher quality of pulp at a minimum of expense fOr installation, upkeep and operation of the necessary equipment.-

Various changes may be made in the details of procedure and in the structure em ployed therefor, without departing from the invention or sacrificing any f its advantages. 1 i

I claim:

1. The method of recovering acid in the sulphite pulp process which comprises maintaining a constant pressure in the relief line from the digester, discharging the gases and liquids from the relief line into an acid absorbing medium, and cooling the acid absorb ing medium to a predetermined temperature.

2. The method of recovering acid in the sulphite pulp process which comprises maintaining a constant pressure in the relief line from the digester, discharging the gases and liquids from the relief line into an acid absorbing medium, cooling the acid absorbing medium to a predetermined temperature, and maintaining the acid absorbing medium under pressure.

3. The method of recovering acid in the sulphite pulp process which comprises maintaining a constant pressure in the relief line common to a plurality of digesters, and discharging the gases and liquids from the re lief line into an acid absorbing medium above atmospheric.

4. The method of recovering acid in the sulphite pulp process which comprises maintaining a constant pressure in the relief line common to a plurality of digesters, discharg-.

ing the gases and liquids from the relief line into an acid absorbing medium, and mamtannng the acid absorbing medium under pressure above atmospheric.

5. The method of recovering acid in the sulphite pulp process which comprises maintaining a constant pressure in the relief line common to a plurality of digesters, discharging the gases and liquids from the relief line into an acid absorbing medium, and cooling the acid absorbing medium to a predetermined temperature above atmospherlc.

6. The method of recovering acid in the sulphite pulp process which comprlses maintaining a constant pressure 1n the relief line common to a plurality of digesters, disch arging the gases and liquids from the relief line into an acid absorbing medium, cooling the acid absorbing medium to a predetermined temperature, and maintaining the acid absorbing medium under pressure above atmospheric. 7. The method of recovering acid in the sulphite pulp process which comprises maintaining a constant pressure in the relief line from the digester and discharging the gases and liquids from the relief line into an acid absorbing medium, and bypassing the blowdown gases around the pressure control means and directly to afresh supply of the acid absorbing medium above atmospheric.

8. The method of recovering acid in the sulphite pulp. process which comprises maintaining a constant pressure in the relief line I pheric. p v

9. The method of recovering acid in the sulphite pulp process which comp-rises absorbing the gases and liquids discharged from the relief line of a digester into an acid absorbing medium under pressure and maintaining the acid absorbing medium at a re duced temperature by cooling.

5 medium being maintained substantially consulphite pulp process 'which comprises absorbing the gases and liquids discharged from the relief line of a digester into an acid absorbing medium under pressure and maintaining the acid absorbing medium at a re duced temperature by circulating Water therethrough.

11. The method of recovering acid in the sulphite process which comprises discharging the gases and liquids from the relief line of the digester into an acid absorbing medium until a predetermined pressure is attained, the temperature of the acid absorbing medium being maintained substantially constant.

12. The method of recovering acid in the sulphite process which comprises discharging the gases and liquids from the relief line of the digester into an acid absorbing medium until a predetermined pressure is attained, the temperature of the acid absorbing stant, and utilizing the acid absorbing medinm in the -digester at the strength determined by the constant temperature and predetermined pressure.

13. In an apparatus for manufacturing I paper pulp, the combination of a digester, an

accumulator, a relief line connecting the diaccumulator, a relief line connecting the digester and accumulator,a recovery tank connected to the accumulator, and means for maintaining a constant pressure in the relief line and accumulator.

14. In an apparatus for manufacturing paper pulp, the combination of a digester, an

gester and accumulator, a recovery tank connected to the accumulator and a pressure reducing valve between'the accumulator and the recoverytank to maintain automatically a constant pressure in the relief line and accumulator.

15. In an apparatus for manufacturing paper pulp, the combination of a digester, an accumulator, a relief line connecting the digester and accumulator, a recovery tank connected to the accumulator, and means for cooling the liquid in the recovery tank to a predetermined temperature.

16. In an apparatus for manufacturing paper pulp, the combination of a digester, an accumulator, a relief line connecting the digester and accumulator, a recovery tank con-' nected to they accumulator, and means for cooling the liquid in-the recovery tank to a predetermined temperature, including a cooling coil invthe recovery tank and means for and accumulator, and means for cooling the liquid in the recovery tank to a predetermined temperature.

18. In an apparatus for manufacturing paper pulp, the combination of a digester, an accumulator, a relief line connecting the digester and accumulator, a recovery tank connected to the accumulator, means including a pressure reducing valve for maintaining a constant pressure in the relief line and accumulator, and means for cooling the liquid in the recovery tank to a predetermined temperature.

19. In an apparatus for manufacturing paper pulp, a plurality of digesters, a recovery tank, a relief line connecting the digesters and the recovery tank, and means for maintaining a constant pressure above atmospheric in the relief line.

ROBERT B. WIlLF.

controlling the flow of cooling liquid therethrough,

17. In an apparatus for manufacturing paper pulp,the combination of a digester,. an accumulator, a reliefline connecting the digester and accumulator, a recovery tankconnected to the accumulator, means for maintaining a constant pressure in the relief line GERME EGA'HE 0th QQRREGTWN. Patent No, warmer, Granted August re, tree, to

WBERT WQLF.

it is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as ioiiows: Page 4, lines 78, 86,

94, 103, 104, 113,122 and 1-23, ciaims 3, 4, 5, 6, 7 and 8, respectively, strike outthe words "above atmospheric"; same page, lines 74, 8h, 89, 9?, 07, and

p 116, claims 3, 4, 5, 6, 7 and 8, respectively, after the word "pressure" insert the words "above atmospheric"; and that the said Letters Patent shouid be read 'with these corrections therein that the same may conform to the record oi the case in the Patent @tfiice. t

Signed and sealed this Zhst day oi October, A. D. 193%.

I p M. Ii. Moore, I (Sear) Acting Commissioner of Patents. 

